Transglutaminases are enzymes that occur ubiquitously in eukaryotic cells, as well as in many extracellular regions. Although they vary significantly in molecular form, they catalyze a single covalent modification reaction, the outcome of which is the permanent attachment of certain protein molecules to one another subsequent to the assembly of their polypeptide chains. The importance of this post-translational event, which occurs through so called epsilon (gamma-glutamyl) lysine or bis-(gamma-glutamyl) polyamine crosslinks is evident in fibrin clot stabilization in hemostasis, vaginal plug formation as a result of postejaculatory clotting of seminal plasma, and production of the cell envelope of the stratum corneum during terminal differentiation of keratinocytes in the epidermis. Each of these reactions is catalyzed by a different transglutaminase and the characteristics of each reflects the individual specificity of the enzyme involved. The purposes of this project are to gain understanding of the molecular basis for specificity differences among the transglutaminases, to construct specific inhibitors for the various enzymes based on this knowledge of specificity differences, and to apply these inhibitors as a means of determining further biological roles for the transglutaminases. Methods have been developed for detecting specificity differences for lysine residues and preliminary tests are encouraging. A number of inhibitors for transglutaminases are under study and isothiocyanates seem applicable.